High Dynamic Range (HDR)

High Dynamic Range (HDR) is a method of capturing and displaying images with much brighter highlights and more detail in darker areas.

For some time, television cameras and film have been able to capture a lot more detail than a TV screen can display. The recently published HDR standards allow new brighter TVs to display these images without compromising the reproduction of details in the highlights or the shadow areas of an image. HDR for television is not the same as you may have on your mobile phone where several different exposures are used to make a single HDR image, although some television cameras use similar techniques to boost their range.

HDR Capture

HDR images can be generated in different ways. Many cameras exploit the ability of the sensor to capture a wide range of brightness and use an HDR “gamma” curve to compress the light range into the electronic signal.

The RED Epic and Scarlet RED for example, use a techniques where the HDR image is created from a combination of two recordings of different exposures at the same time. It will capture firstly the normal standard aperture and shutter settings in one image, while simultaneously recording a second exposure for highlight protection, using an adjustable shutter speed that is 2-6 stops faster.

Special tests were performed with the ARRI Alexa in 3D recording rigs. The 3D rig wasn’t used to capture stereoscopic images, but was instead used to film the same image simultaneously, in synchronous, using different exposures to produce extremely high dynamic image data that can be merged and controlled in the grade or colour correction process.

No matter which process is used the finished image is processed using one of the international HDR gamma curves before being sent to the display.

HDR Monitoring

Most displays are unable to reproduce high contrast images and therefore there is a problem of monitoring HDR content especially on location.  Displays that can only handle 8-bit (256 gradations from black to white) will not reproduce a usable image if fed with an HDR signal.  However most broadcast displays have 10-bit (1024 graduations) processing that will do a better job if a tone mapping method is used to compress the dynamic range. 

Tone mapping via a look-up table (LUT ) is an attempt to scale the HDR input image by compressing and limiting the signal to match the particular display.

HD is designed to work on displays with a peak brightness of 100cd/m².  Current HD flat screen displays often reach 250 to 300cd/m² but this is not HDR, these displays are simply stretching the HD image and this often shows as stepping (contouring) between brightness levels. New HDR displays capable of peak brightness that goes far beyond the HD standard ITU-R BT.709 are in development and typically reach between 600 and 2000cd/m² and there are experimental displays that achieve between 4000 and 10000cd/m². 

HDR displays use the new expanded colour pallet standard ITU-R BT.2020 , often referred to as Rec.2020.  With growth in the audience appetite for UHD, HDR will play a much greater role in films and new types of output. 

Progress is rapid. The technology is slowly gaining market maturity and corresponding workflows are being integrated into post-production pipelines. Both software vendors as well as professional and home display manufacturers have already implemented some HDR functionality. Just as the ITU-R BT.2020 standard that governs UHD TV format, the recently published ITU-R BT.2011 details the image parameters for the production, broadcasting and display of HDR content. In addition to those included in the UHD standard definitions for screen sizes, frame rates and aspect ratio, BT.2100 specifies a peak reference luminance of 1000 cd/m² and a black level value (minimum luminance) of 0.005cd/m².